In experiments, some phenomena have been derived form the operator's open-loop frequency response when a random exponential signal generated by a simple linear system has been used as a reference input. These phenomena cannot be described by any usual isomorphic models. As a parameter included in the reference input generator has changed, these phenomena have changed their features in such a complicated manner that usually interpretations of human input adaptability are no longer useful. It is shown that the phenomena are closely related to the inherent dead time included in the human operator and can be represented systematically by introducing an algorithmic model that realizes output regulation with internal stability for linear systems with a dead time